The significance of retrograde axonal transport to the visual system will be studied. Purified horseradish peroxidase, an enzyme tracer frequently used in neuroanatomical studies, will be modified chemically and tested morphologically to ascertain which groups on this molecule determine the specificity of uptake and retrograde transport by visual system axons. This information will be extrapolated to study the biochemical properties of endogenous substances synthesized in the neuronal somata and transported orthogradely and retrogradely in axons, and also exogenous proteins pinocytosed and transported retrogradely in axons. The significance of the transported materials in the neuronal response to trauma and during development will be examined. A new neuroanatomical tracing technique based on retrograde axonal transport will be developed for use in demonstrating axonal collaterals.